1. Field of the Invention
The present invention relates to an autofocusing motion picture camera having an adjustable focus zoom lens and having means for periodically determining the distance to subjects to be photographed in general, and to such a camera having a highly directional, object presence detecting transducer, in particular.
2. Description of the Prior Art
Photographic cameras incorporating control systems that automatically position the movable element of an adjustable focus lens of said camera to a position where an image of a remote object is in focus at the image plane of said camera in response to a signal representative of the distance to said remote object, have been disclosed in the prior art. Control systems of this type that include object sensing, highly directional transducers for sensing the presence of objects to be photographed remote from said photographic camera are also known in the prior art.
The lens of a camera, as well as other optical instruments has a characteristic viewing angle associated with same that spans the area that is visible through said lens and is generally referred to as the lens' optical field of view angle. In an adjustable focus zoom lens, this characteristic viewing angle is adjustable between a wide angle position at one end of its zooming range where said viewing angle is relatively wide and a telephoto position at the other end of its zooming range where said viewing angle is relatively narrow. Similarly, a highly directional, object presence detecting transducer for use in a control system in an autofocusing camera of the type described above also has a characteristic sensing angle associated with the transducer that spans a region within which the presence of an object can be detected and that angle is referred to herein as the transducer acceptance or main lobe angle.
When selecting the size of the transducer acceptance angle for the adjustable focus lens control system transducer, normal practice is to select an angle that is substantially smaller than the field of view angle of its associated adjustable focus lens, said acceptance angle usually being in the neighborhood of 10% of said field of view angle. If the acceptance angle of the control system transducer was greater than the field of view angle of the adjustable focus lens that it helps to focus, a spurious object presence detection signal would be generated by objects located outside of the adjustable focus lens optical field of view which were closer to said control system transducer than objects located within said optical field of view, which might result in a misfocused adjustable focus lens. This problem is avoided by selecting an object presence sensing transducer with a relatively narrow acceptance angle in accordance with the criteria mentioned above.
In a zoom type adjustable focus lens, the optical field of view of said lens is adjustable between wide angle and telephoto positions, having relatively wide and narrow optical fields of view angles, respectively, as previously mentioned. Standard design practice, as discussed above, would dictate that the transducer acceptance angle for the adjustable focus lens control system transducer should be in the neighborhood of 10% of the narrowest (telephoto) field of view angle of said zoom type adjustable focus lens to avoid the spurious signal problem mentioned above.
One disadvantage with requiring that a transducer acceptance angle be in the neighborhood of 10% of the field of view angle of the adjustable focus lens that it helps to focus is that a zoom type adjustable focus lens would require a transducer with a variable acceptance angle. While it is possible to vary the acceptance angle of a transducer, it would normally result in a more complex arrangement than would be necessary if the transducer acceptance angle remained fixed.
Another disadvantage in selecting a transducer acceptance angle for a transducer in a zoom type adjustable focus lens control system that is in the neighborhood of 10% of the narrowest field of view of said lens is that such an acceptance angle would be substantially less than 10% of the widest (wide angle) field of view of said lens which might result in delayed focusing or the misfocusing of said zoom type adjustable focus lens when the wide angle zoom lens position is selected. This might occur because of a delay or the inability to sense objects within the wide angle field of view of said lens.